1. Field of the Invention
The present invention generally relates to coating processes. More particularly, this invention is directed to a vapor deposition process and apparatus for depositing ceramic coatings containing multiple oxides with different vapor pressures using a single evaporation source containing the multiple oxides.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, certain components of the turbine, combustor and augmentor sections of a gas turbine engine can be required to operate at temperatures at which the mechanical properties of such alloys would be insufficient. For this reason, these components are often protected by a thermal barrier coating (TBC) formed of a ceramic material. Because of the different coefficients of thermal expansion (CTE) between ceramic materials and the superalloy substrates they protect, an oxidation-resistant bond coat is typically employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
Various ceramic materials have been proposed for TBC""s, the most notable of which is zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another alkaline-earth metal oxides, or ceria (CeO2) or another rare-earth metal oxides. Binary yttria-stabilized zirconia (YSZ) is widely used as a TBC material because of its high temperature capability, low thermal conductivity and erosion resistance in comparison to zirconia stabilized by other oxides. YSZ is also preferred as a result of the relative ease with which it can be deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency. As a result, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
To reduce and stabilize the thermal conductivity of YSZ, ternary YSZ systems have been proposed. For example, commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a TBC of YSZ alloyed to contain certain amounts of one or more alkaline-earth metal oxides (magnesia (MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (lanthana (La2O3), ceija (CeO2), neodymia (Nd2O3), gadolinium oxide (Gd2O3) and dysprosia (Dy2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al., when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. In commonly-assigned U.S. patent application Serial No. 10/064,785 to Darolia et al., a TBC of YSZ is deposited to contain a third oxide, elemental carbon and potentially carbides and/or a carbon-containing gas. The resulting TBC is characterized by lower density and thermal conductivity, high temperature stability and improved mechanical properties.
While the incorporation of additional oxide compounds into a YSZ TBC in accordance with Rigney et al. and Darolia et al. has made possible a more stabilized TBC microstructures, it can be difficult to deposit a TBC by an evaporation process to produce a desired and uniform composition if the additional oxide has a significantly different vapor pressure than zirconia and yttria. For example, ceria has a vapor pressure of about 10 mbar, in comparison to vapor pressures of about 0.05 mbar for zirconia and yttria at 2500xc2x0 C. If a YSZ+ceria TBC is to be deposited by EBPVD or another vapor deposition process, evaporating a single ingot containing the desired YSZ+ceria composition deposits a TBC that has an unacceptable nonuniform distribution of ceria. To avoid this result, co-evaporation of oxides having vapor pressures significantly different from YSZ (e.g., at least an order of magnitude higher than YSZ) has been performed with a separate ingot of each additional oxide. If a single electron beam is used, a controlled beam jumping technique must be employed, by which the beam is briefly projected (in the millisecond range) on each ingot, with the amount of time on each ingot being adjusted so that the energy output achieves the energy balance required to obtain compositional control. As an alternative to the use of a single beam, multiple electron guns operated at different power levels have been used to maintain molten pools of each ingot material. However, both of these techniques complicate the deposition process such that the incorporation of additional oxides in a YSZ TBC can be difficult to perform in a commercial setting.
In view of the above, it would be desirable if a process existed that simplified the co-evaporation of oxides with different vapor pressures.
The present invention provides a process and apparatus for depositing a ceramic coating, such as a thermal barrier coating (TBC) for a component intended for use in a hostile thermal environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The process of this invention is particularly directed to an evaporation technique for depositing a TBC formed of multiple oxide compounds, at least one of which has a vapor pressure that differs from the remaining oxide compounds. An example is in the deposition of a TBC formed of YSZ alloyed with a third oxide to reduce the density and/or thermal conductivity of the TBC, improve high temperature stability, and/or improve mechanical properties.
The invention generally entails providing an evaporation source containing multiple different oxide compounds, at least one of the oxide compounds having a vapor pressure that is higher than the remaining oxide compounds. In a YSZ coating system, examples of particularly suitable oxide compounds are metal oxides of metals such as cerium, gadolinium, neodymium, lanthanum, dysprosium, ytterbium, tantalum, magnesium, calcium, strontium and barium, which have a sufficient absolute percent ion size difference relative to zirconium ions to produce significant lattice strains that promote lower thermal conductivities. The component intended to be coated is suspended near the evaporation source, and a high-energy (e.g., electron or laser) beam is projected onto the evaporation source to melt and form a vapor cloud of the oxide compounds of the evaporation source, while preventing the vapor cloud from contacting and condensing on the component during an initial phase in which the relative amount of the one oxide compound in the vapor cloud is greater than the relative amount of the oxide compound in the evaporation source. For this purpose, a barrier may be physically placed between the component and the evaporation source. During a subsequent phase, in which the relative amount of the oxide compound in the vapor cloud has decreased to something approximately equal to its relative amount in the evaporation source, the vapor cloud is allowed to contact and condense on the component to form the coating. If a barrier was used to initially prevent deposition of the coating, the barrier is removed during this subsequent phase of the evaporation process.
In view of the above, it can be appreciated that the present invention is based on a determination that, at the beginning of an evaporation process using an evaporation source (e.g., ingot) containing multiple oxide compounds including one or more whose vapor pressure is higher than the other oxide compounds, the vapor cloud is rich with the oxide compound with the highest vapor pressure, as a result of the oxide compound evaporating faster than the other oxide compounds. Furthermore, it was determined that over a period of time, the evaporation source becomes enriched in the oxide component(s) having lower vapor pressures (corresponding to lower evaporation rates), with the result that an equilibrium (or near equilibrium) is established in the evaporation process, resulting in a more uniform co-evaporation of the oxide compounds in the evaporation source. As a result, a coating deposited during this phase of the evaporation process will have a composition more nearly equal to that of the evaporation source. Accordingly, a preferred aspect of the present invention is to allow the vapor cloud evaporated from an evaporation source to contact and condense on the article primarily or exclusively during this later phase, producing a coating whose composition is more predictable and uniform than otherwise possible when using a single evaporation source for the multiple oxide compounds.
Other objects and advantages of this invention will be better appreciated from the following detailed description.